It could still be placebo effect and/or whatever makes purple/red grapes their deep color having an influence on the flavor and this being common to both purples rather than spilled Fanta. The article mentions kudzu which is also purple and allegedly vaguely grape like.

Basically "this bloody mary sure does have a hint of pizza to it" but one level lower (chemical level rather than the tomato ingredient level).

Kudzu flowers supposedly have a grape like flavor. I just had a beer made with them from fonta flora in NC and you could vaguely taste it. Although unsure if it was my imagination.

Kudzu flowers were listed as one of the possibilities in the article.